Rare muon pairs made by chance or enhanced?

The rare decay of a Bs0 meson into two muons would be greatly enhanced by some models of new physics.

While the Standard Model allows the Bs0 meson, the particle containing a strange quark and an antibottom quark, to decay into two muons, the chance of that happening is about half that of a given lottery ticket winning the Mega Millions jackpot. This rare process is more common in theories with new physics, such as supersymmetry, where the presence of new particles can significantly enhance the Bs0 decay rate into muon pairs. The search for this decay provides an excellent window for finding any new physics that would alter the decay rate and has been a key part of the worldwide effort to search for physics beyond the Standard Model for many years. A new analysis at DZero significantly improves upon the previous DZero sensitivity to this decay and complements recent results from other collaborations.

Because this decay process is so rare and the aim of the analysis is to be sensitive to just a very few events, the analysis itself is performed “blindly,” or without looking at the signal region until the entire analysis is otherwise finalized. There are two major backgrounds to the signal in this search, and the analyzers used a two-pronged approach to mitigate them. Since each major background involves more than one particle decaying in order to produce the two muons, the analyzers used information about the activity around each muon in the event to help classify it as signal or background. The analyzers also created two discriminants, one to separate the signal from each source of background, and then determined the optimal cuts on each to use as the final signal selection.

After examining the data in the signal region, the DZero analyzers find no significant excess over the background. They set a limit on the chance of a Bs0 decaying into two muons to be less than about 15 times for every billion decays, which is nearly 3½ times better than the previous DZero result. While still not sensitive enough to measure the expected Standard Model signal, the DZero constraint is the strongest limit from the Tevatron on new physics in this rare decay. The result is in agreement with the most recent results from the CDF, ATLAS, CMS and LHCb collaborations.

Mike Cooke

These physicists made major contributions to this analysis.
When a quark is produced as part of a particle collision, it turns into a spray of particles called a jet. The b-jet identification group provides tools that separate jets that originate from a bottom quark from those that originate from lighter quarks, a key element to many physics analyses at DZero.